Power equipment using high-temperature superconductors such as a generator, a motor, a potential transformer, and the like operates under a temperature of approximately 30K. Since high refrigeration capacity is necessary to cool the power equipment, a cooling system having a plurality of (approximately five) Gifford-McMahon (GM) refrigerators or pulse tube refrigerators (hereinafter simply called “refrigerators,” except in necessary) is used.
To stably operate the power equipment for a long term, the cooling system for cooling the equipment also has to stably operate for a long term. Accordingly, when the refrigeration capacity decreases due to the deterioration or breakdown of the refrigerator installed in the cooling system, it is preferable that the refrigerator can be exchanged.
FIG. 1 shows an overview of an ordinary cooling system. The cooling system adopts an indirect cooling method, in which when an object 10 to be cooled (for example, power equipment using high-temperature superconductors, for example, a rotor of a generator) is cooled to approximately 30K in the vicinity of which neon liquefies, a neon gas used as a refrigerant gas is once liquefied and retained in a liquefied neon container 12 to cool a helium gas circulating through the object 10 to be cooled in pipes 20. The helium gas circulating through the pipes 20 is sent out by a helium gas circulating pump 22 at ambient temperature, and is cooled by heat exchange with the helium gas returning through a first heat exchanger 24. Then, in a second heat exchanger 26, the helium gas is further cooled to approximately 30K by the liquefied neon retained in the liquefied neon container 12. After passing through a third heat exchanger 28 to cool the object 10 to be cooled, the helium gas gets into the first heat exchanger 24 in which the temperature increases to ambient temperature, and then returns to the circulating pump 22.
A cryostat 30 of the cooling system as a vacuum insulation container is provided with refrigerator attachment sleeves 32 into which a cylinder 44 of a plurality (two in the drawing) of refrigerators 40 and 42, provided to obtain high refrigeration capacity, is just fitted. Though the two refrigerators are provided in the drawing, the number of the refrigerators is changeable to one or more than three.
A refrigerator attachment flange 46, for attaching the refrigerators 40 and 42 to the cryostat 30, is provided with a seal O-ring 48 for the purpose of preventing the neon gas from leaking outside, and preventing air from getting inside. The O-ring is provided in the flange 46 of the refrigerator in the drawing, but may be provided in the cryostat 30. A member for sealing is not limited to the O-ring, as long as the member can seal the gas.
A plurality of (two in the drawing) pipes 14 provided in the liquefied neon container 12 are connected to the refrigerator attachment sleeves 32 disposed above them. The internal diameter of the pipe 14 does not interfere with the circulation of the neon gas.
Since the neon gas is sealed in the liquefied neon container 12, the temperature of a low-temperature cooling stage 50 of the refrigerator (a second cooling stage, hereinafter simply called “cooling stage”) decreases to a condensing temperature (a liquefaction temperature) of neon when the refrigerator operates, so that the neon gas condensing and dropping is retained in the container 12 disposed beneath. A low-temperature portion of 30K which has to be securely insulated from heat is generally disposed inside the vacuum insulation container (the cryostat 30 in the drawing). Incidentally, a vacuum exhaust system is omitted in the drawing.
In the drawing, reference numeral 52 is a compressor of the refrigerator.
Taking a case where one of the refrigerators deteriorates or breaks down due to some reason. In this case, in an ordinary cooling system, the refrigerator deteriorated or broken down in performance cannot be exchanged without stopping the operation of the cooling system to increase the temperature of the whole cooling system. This is because when the refrigerator is detached for exchange under operating condition of the cooling system (the other normal refrigerators keep operating without recovering the neon gas), the neon gas is lost (leaks) from the open container 12 for sealing the neon gas, and air and moisture getting into the container 12 increases temperature, so that it becomes impossible to keep cooling. Accordingly, before the refrigerator is detached, the cooling system is stopped to recover the neon gas (a neon gas recovery system is omitted in FIG. 1) and the temperature of the system increases to ambient temperature.
Although, in Japanese Patent No. 3265139 and Japanese Patent Laid-Open Publication No. Hei. 9-113048, a heat switch for quick pre-cooling is disposed between a high-temperature cooling stage and a low-temperature cooling stage of a refrigerator cylinder, between an object to be cooled and a heat shield for covering a low-temperature portion, or between the object to be cooled and the refrigerator, the cooling system has only one refrigerator, so that it is never considered that one or some of the plurality of refrigerators is detached.